1.e4 e5 is played to stop white from playing yet another pawn to the center with 2.d4. Sicilian Defense (1...c5) also accomplishes this with the c pawn. This way if you end up trading on d4 you can still have both of your central pawns unlike your opponent and have an advantage in the center.
Symmetric positions like 1.e4 e5 2.Nf3 Nf6 usually don't give much of a winning chance to black and tend to be drawish. Sicilian on the other hand creates imbalance in the first move and shows black is here for the win! Imbalance creating openings (Sicilian, Nimzo Indian) become popular for black, otherwise it's hard to get the advantage. White tends to get the initiative early on but the pawn structure suggests black is better if he can survive the early middlegame.
I realize this is a basic explanation and misses lots of exceptions but.. it is just move 1, don't expect too much from me!